The HIV protease enzyme is essential for HIV infection. MK-639 is a specific inhibitor of this enzyme and inhibits HIV in vitro. ZDV in combination with lamivudine has recently been shown to potentially inhibit HIV replication in patients. This study in HIV infected subjects is designed to assess the safety and effect on virus levels of MK-639 given with ZDV and 3TC compared to MK-639 given alone or to ZDV combined with 3TC.